A non-A, non-B antigen will be sought in serum from patients with acute or chronic non-A, non-B hepatitis by reacting them with convalescent phase serum in a solid phase radioimmunoassay. In addition, plasma units derived from: either donors implicated in non-A, non-B hepatitis will be ultracentrifuged. The pellet derived by ultracentrifugation will be reacted against a radiolabelled IgG fraction obtained from patients convalescent from non-A, non-B hepatitis in a solid phase radioimmunoassay system.